Polyurethane is a dense spray injected, expanding foam insulation. As it is sprayed in the door from both sides, it expands to fill every nook and cranny inside the door, which adds rigidity. Polyurethane is only available for 3 layer garage doors. On average, it offers an R-value of R12–R19. Polyurethane is good for extreme climates where the temperature outside can greatly affect the temperature inside of your garage.